The present invention relates to increasing the capacity of a cellular system, and more specifically, to increasing the capacity of a cell without causing an increased amount of interference to connections in the system.
Continuing growth in telecommunications is placing increasing stress on the capacity of cellular systems. The limited frequency spectrum made available for cellular communications demands cellular systems having increased network capacity and adaptability to various communications traffic situations. Although the introduction of digital modulation to cellular systems has increased system capacity, these increases alone may be insufficient to satisfy added demand for capacity and radio coverage. Other measures to increase capacity, such as decreasing the size of cells in metropolitan areas, may be necessary to meet growing demand.
Interference between communication cells located near one another creates additional problems, particularly when relatively small cells are utilized. Thus, techniques are necessary for minimizing interference between cells. One known technique used in TDMA and FDMA systems is to group cells into xe2x80x9cclustersxe2x80x9d. Within individual clusters, communications frequencies are allocated to particular cells in a manner which attempts to maximize the uniform distance between cells in different clusters which use the same communications frequencies. This distance is commonly referred to as the xe2x80x9cfrequency reusexe2x80x9d distance. As this distance increases, the interference between a cell using a communication frequency and a distant cell using that same frequency is reduced.
Another method of increasing capacity while reducing interference is through the use of spread spectrum modulation and code division multiple access (CDMA) techniques. In typical direct sequence CDMA systems, an information data stream to be transmitted is superimposed on a much-higher-symbol-rate data stream, sometimes known as a spreading sequence. Each symbol of the spreading sequence is commonly referred to as a chip. Each information signal is allocated a unique spreading code that is used to generate the spreading sequence typically by periodic repetition. The information signal and the spreading sequence are typically combined by multiplication in a process sometimes called coding or spreading the information signal. A plurality of spread information signals are transmitted as modulations of radio frequency carrier waves and are jointly received as a composite signal at a receiver. Each of the spread signals overlaps all of the other coded signals, as well as noise-related signals, in both frequency and time. By correlating the composite signal with one of the unique spreading sequences, the corresponding information signal can be isolated and decoded. Since signals in CDMA systems overlay one another in frequency and time they are frequently said to be self-interfering.
One method of reducing self-interference in a CDMA cellular system is through the use of power control. Power control in cellular systems is based upon the premise that as the distance between the mobile station and the base transceiver station decreases, the amount of transmit power needed for the mobile station or the base transceiver station to receive an acceptable signal will also decrease. Similarly, as the distance between the base transceiver station and the mobile station increases, the amount of transmit power needed for the mobile station or the base transceiver station to receive an acceptable signal will also increase. As the magnitude of the transmit power is increased, so to does the amount of interference caused to other connections in the cellular system. Accordingly, by using only the amount of power necessary to transmit signals between base transceiver stations and mobile stations, the amount of interference caused to other connections in the system will be decreased.
FIG. 1 illustrates another method used to reduce interference in a CDMA system. Cells A, B and C spread communication signals over a first frequency band f1. The cells overlap each other at the shaded regions 140 and 150 so that there are minimal interruptions to an ongoing call during handover. Accordingly, when mobile station 110, which is communicating with a base transceiver station in cell A over frequency band f1, moves from an area completely contained within cell A to shaded region 140, the connection between mobile station 110 and cell A will cause interference to connections in cell B, which also are communicating on frequency band f1, until a connection is also establish to cell B. Transferring connection between cells which operate over the same frequency band is known as soft handoff.
Now consider a situation wherein, after the cellular system has been implemented, it is discovered that there is an increased demand for access to the channels allocated to cell B which, in turn, leads to an unacceptable level of interference. The area where the increased demand occurs is referred to in the art as a xe2x80x9chot spotxe2x80x9d. To reduce the interference associated with a highly loaded cell, a second frequency band f2 can be assigned to the transmitter in cell B so that the transmitter in cell B can communicate with mobile stations on either frequency band f1 or on frequency band f2. Accordingly, when the system detects an increase in the load on frequency band f1, which the system determines will lead to an unacceptable level of interference, the system can transfer some of the mobile stations over to frequency band f2. Typically the determination of whether an increase in the load will lead to an unacceptable level of interference can be based on a predefined number of users on a particular frequency band, if the total output power used by the system exceeds a predetermined threshold, or if the total uplink interference caused by the mobile stations exceeds a predetermined threshold.
For example, assume that cell B is communicating with mobile stations on both frequency band f1 and frequency band f2, and cell A is communicating with mobile stations only on frequency band f1. Assume further that mobile station 110 is communicating on frequency band f1 with a base transceiver station in cell A and that frequency band f1 in cell B is becoming congested. As the mobile station 110 moves further into the area of coverage of cell B and away from the area of coverage of cell A, the mobile station 110 or the cellular system will determine that mobile station 110""s signal quality can be improved and the amount of interference cause to other mobile stations reduced if a connection between the mobile station 110 and a base transceiver station in cell B on frequency band f2 is established. However, before the connection is handed off, mobile station 110 will cause interference to the mobile stations in cell B, since both mobile station 110 and the mobile stations in cell B will be transmitting over the same frequency band, i.e., frequency band f1. Accordingly, although congestion is relieved in cell B, interference will still be caused to mobile stations in cell B which are operating on frequency band f1.
Another alternative method for increasing system capacity while minimizing interference is through the use of localized microcells which may be established within overlying macrocells to handle areas with relatively dense concentrations of mobile users. Typically, microcells may be established for thoroughfares such as crossroads or streets, and a series of microcells may provide coverage of major traffic arteries such as highways. Microcells may also be assigned to large buildings, airports, and shopping malls. Microcells allow additional communication channels to be located in the vicinity of actual need, thereby increasing cell capacity while maintaining low levels of interference.
Implementation of microcells within a macrocell typically requires the use of separate frequencies for communication on the channels assigned to the microcell and for the channels assigned to the macrocell. Further, implementation of microcells within a macrocell requires separate transmitters, i.e., base transceiver stations, for communications on the channels assigned to the microcell and for channels assigned to the macrocell. These microcell transceivers typically have lower maximum transmit powers than macrocell transceivers and, accordingly, create relatively less interference through their transmissions. Although the use of microcells may reduce interference, the use of microcells also increases the cost of providing the additional channels by requiring the installation of additional transmitters and through increased costs of cell planning due to the complexity which results from the use of microcells. Further, since the transceivers for a microcell are not usually located in the same geographic area as the transceivers for the macrocells, there are increased maintenance costs associated with the geographic separation. In addition, although microcells may reduce the load on the macrocell and reduce the average power levels used by the mobile stations in the microcell, the microcell may also have to tolerate high levels of interference.
Accordingly, it would be desirable to increase the capacity of a cellular communications system without increasing excess interference to the existing connections in the cellular system. Further, it would be desirable to increase capacity in a cellular system without adding extra base transceiver stations and the associated added expenses. In addition, it would be desirable to allow a handoff into a cell with increased capacity which does not cause excess interference towards the existing connections in the cell.
These and other problems associated with cellular communications are solved by the present invention, wherein a base transceiver station which communicates with mobile stations over a first and second frequency band uses a tailored range for the second frequency band in order to minimize interference. According to one embodiment of the present invention, the maximum range for the second frequency band is smaller than the maximum range for the first frequency band. According to another embodiment of the present invention, the maximum range for the second frequency band is greater than the maximum range for the first frequency band. According to yet another embodiment of the present invention, the maximum range of the second frequency band will vary depending upon the congestion of the first frequency band.